Cocktail denotes the length of the dress (to the knee or a little shorter), plus the assumption that it's a time period where cocktails would be served (like 4-7pm)...so whatever you would wear that's a bit fancier than afterwork but not evening wear. Casual should denote (especially) the footwear being more casual for women, men not needing a sport coat necessarily (but prob a tie) and less wool pants--more like khakis or cotton/twill pants.
The best thing to aim for would be a v-neck sheath dress in a fun/slimming color (like Navy blue or chocolate brown) with a light wrap or cardigan if you're self concious about your arms (I am!) and some light/minimal jewelry. I think a heeled sandal would look great for shoes...no hose required!
In brown w/ jewelry to dress it up & a comfy heel:
I just bought this and it's smokin' hot: